1. Field of the Invention
This invention relates to a diaphragm-type sensor for use in a flow detecting apparatus.
2. Description of the Prior Art
FIG. 1 is a perspective view of a conventional flow sensor of a type known as a microbridge flow sensor. The flow sensor as illustrated comprises a semiconductor substrate 1 made, for example, of silicon which is formed with a cavity 4 at a central portion thereof communicating openings 2, 3 at both side portions. A bridge 5 is formed over the cavity 4 so as to be thermally insulated from the semiconductor substrate 1. The bridge 5 includes a thin-film heater element 7 and two thin-film thermally sensitive resistor elements 8, 9 aligned on both sides of the heater element 7, the three elements being formed by a known thin-film forming technique. The semiconductor substrate 1 further comprises another thin-film thermally sensitive resistor element 10 formed at a corner portion thereof. A slit-like central opening 11 is also formed in addition to the side openings 2, 3 such that portions exposed by these openings 2, 3 and 11 are etched by an anisotropic etchant such as KOH to form the cavity 4 in an inverted trapezoidal shape as well as the bridge 5 supported by the semiconductor substrate 1, wherein the heater element 7 and the thermal sensor elements 8, 9 are thermally insulated from the semiconductor substrate 1 by the cavity 4. The elements 7, 8 and 9 are further encapsulated by a protective layer (6 in FIG. 2) made of a material having a low thermal conductivity ratio such as silicon nitride.
FIGS. 2a and 2b shows the operation of the microbridge flow sensor shown in FIG. 1. FIG. 2a illustrates a temperature distribution of the respective elements 7, 8 and 9 while FIG. 2b a cross-sectional view of the sensor taken along a line B--B' of FIG. 1.
If the heater element 7 is controlled to remain at a constant temperature higher than an ambient temperature, for example 63.degree. C., the thermal sensor element 8, 9 exhibit a substantially equal temperature as shown in FIG. 2a, for example, 35.degree. C. In this circumstance, if a fluid flows in the direction indicated by an arrow 12 shown in FIG. 1, the upstream sensor element 8 is cooled down by a temperature .DELTA.T.sub.3 while the downstream sensor element 9 is heated by .DELTA.T.sub.4. Consequently, a temperature difference is produced between the upstream sensor element 8 and the downstream sensor element 9. Thus, the microbridge flow sensor is used for detecting flow rates by incorporating the thermal sensor elements 8, 9 in a Wheatstone bridge to convert the temperature difference to a voltage signal corresponding to a flow rate of a fluid under measurement.
The microbridge flow sensor, as mentioned above, has a thin film bridge structure with an extremely small thermal capacitance formed by the thin film technique and the anisotropic etching technique and is advantageous in a very high response speed, a high sensitivity, a low power consumption, a good adaptability to a mass production, and so on.
However, the conventional microbridge flow sensor includes relatively large openings 2, 3 on both sides which are necessarily formed for etching semiconductor substrate 1 to form the cavity 4. The large openings 2, 3 causes a fluid flow to go into the cavity 4, whereby dust in the fluid may be attached in vicinity of the openings or in cavity 4 to badly affects the sensor characteristics.